This invention relates to radio frequency taps and chokes, and more particularly to a device for separating a single phase AC power signal from a broadband signal carried on the same conductor that has improved power consumption characteristics.
When distributing radio frequency (RF) signals, such as television signals, over cable, it is common practice to transmit RF signals and single phase AC power signals over the same coaxial cable simultaneously.
These RF signals originate from a central location commonly referred to as the "headend". The media used to carry the RF signals, typically a coaxial cable, inherently has loss characteristics. Thus an amplifier station must be installed at appropriate locations along the cable to compensate for the losses and deliver the RF signal levels as closely as possible to what they were at the headend. The single phase AC power signal, which in the United States is conventionally 60 Hz, is needed to operate the amplifier stations.
The peak power signals are passed along the cable concurrently with the RF signals. The power level of the AC signal is typically 50,000 times greater than that of the RF signal, and uses different and separate circuitry to operate the amplifier station. Therefore, the AC power signal must be separated from the RF signal at each of the amplifier stations.
Other equipment, in addition to the aforementioned stations, is used in cable distribution for distributing RF signals according to subscriber requirements. This other equipment such as passive equipment, do not need the single AC power signals for its operation. The passive equipment, however, must be able to pass and distribute the AC power signal without interfering with the various operations that relate to the RF signals.
In the amplifier stations which must use the single phase AC power signal and in the passive equipment which must pass or distribute the AC power signals, special circuits are employed for separating the RF signals from the AC power signal. Also, the equipment used in the cable system for introducing, or coupling, the single phase AC power signal into the system employs special circuits, similar to the separating circuits mentioned above, which operate to combine the AC power signal and the RF signals. The special circuits will hereinafter be collectively referred to as the frequency duplexing circuits for clarity of this description.
One of the main problems with cable systems results from cascading of the many similar circuits used in the equipment provided along the length of the cable system. Each piece of the various types of equipment will have a characteristic frequency response, and it is desirable that each piece of equipment be capable of maintaining the relative level of all of the RF signals to each other. That is, the relative levels of the RF signals at the output end of each piece of the equipment should ideally be identical with the relative levels at the input end. This is often referred to as a "flat" response and means that the equipment is not contributing unwanted variations in the signal levels regardless of the frequency of the signal. In actuality, circuits do not have perfectly "flat" frequency responses and degradation of the "flat" response becomes a bigger problem at higher frequencies and as the bandwidth of frequencies increases. In equipment of the same make, flatness degradations are usually of the same type and occur at about the same spot in the RF bandwidth. When the equipment is cascaded, the flatness degradations are cumulative and cause what is called a "signature". If the cascade is long and the flatness degradation of a single unit is large enough, the end-of-the-line flatness degradation will be unacceptably high causing severe deterioration of signal quality. Therefore, one of the objectives in equipment design is to keep flatness degradations to a minimum.
The frequency duplexing circuits used in the hereinbefore described cable equipment are by function and necessity in the main RF signal path of the cable system, and radio frequency (RF) chokes are the primary components in these frequency duplexing circuits because all the single phase AC current passes through them and they are connected directly to the RF signal path.
As is well known, an RF choke 36 such as the one shown in FIG. 2, is an inductor which exhibits a high reactance or impedance to signals in the RF frequency range and low impedance to signals of lower frequency. This choke 36 has a plurality of 11 windings of a conductor (20 AWG wire) tightly wound upon a core of ferrite material 14. A resistor, preferably 510 ohms is connected between a first of these windings and winding number 4. In cable systems today, the frequency range of RF signals is from about 5 to 750 MHz. The RF chokes employed as described above in the cable transmission and distribution systems presents a high impedance to those frequencies of the RF signals, and offers virtually no impedance to the lower frequency single phase AC power circuit. This inherent characteristic of RF chokes makes them useful in the separation and combining of RF signals and single phase AC power signals. For example, if such an RF choke were connected with one end tied to the main line carrying both RF signals and AC power signals, and the other end tied to an AC input of a power supply, its function, ideally, would be to provide a low impedance path for the AC power signals to the power supply while presenting a very large impedance to the high frequency radio frequency signals. The result would be that the AC power signal is diverted to the power supply while the RF signals would continue completely unaffected. It should be stated that the separating function described above can only be fully effected by the RF choke in conjunction with other components of the circuit.
Traditional RF chokes, unfortunately, do not offer a uniformly high impedance to all frequencies in the bandwidth of RF signals from 5 to 500 MHz. As is known, most so called traditional RF chokes consists of several turns of insulated wire wound around a ferro-magnetic core. In cable systems, in order to maintain a sufficiently high inductive reactance, or impedance at the 5 MHz end of the frequency band, the RF chokes must have a relatively large number of turns of wire. Due to the physical configuration of these RF chokes, parasitic capacitances exist between the windings of the coil. These capacitances in conjunction with the inductance of the coil form parasitic resonances, most of which are series resonances. The presence of series resonances, along portions of the RF choke, cause significant reductions in its impedance at the resonant frequencies. The Q of these resonant circuits is high enough to cause significant and oftentimes sharp degradations in equipment flatness, and the RF signals are undesirably affected thereby.
Traditional chokes of the type described briefly above can perform well in cable systems having an upper frequency limit of approximately 500 MHz. However, increasing usage of such cable systems results in the need for wider bandwidths and these traditional RF chokes simply do not perform well at higher frequencies.
In addition to the effects on frequency response, the RF chokes used in the equipment of the cable system must be capable of passing several amperes of AC current. The wire used for the coil must therefore be large enough to carry relatively high currents, usually up to about 14 amperes in such cable transmission systems, without becoming excessively warm. Unfortunately, the larger the wire size the more troublesome is the parasitic resonance problem. High currents also pose problems in that core materials are likely to approach saturation thereby presenting the RF signals with an impedance which varies at the frequency rate of the single phase AC power signal. The effect of this is the unwanted modulation of RF signals and this problem is commonly referred to as "hum mod".
The above described problems due to high AC current can be effectively reduced by careful selection of wire size, core material, and core geometry. Many RF chokes have been used to give good performance to the 5 MHz to 450 MHz frequency range. However, when these chokes are used for the 5 MHz to 1,000 MHz frequency range, these chokes have the drawback that they have a moderate amount of insertion loss at frequencies above 750 MHz. These insertion losses cascaded over many circuits results in substantial RF distortion. Further, it is desirable to maximize the reduction of insertion losses as many chokes are cascaded over large networks. Thus, savings of even the smallest amount of insertion loss manifests into a substantial amount of power savings over a large network.
Cable system capabilities are needed for extended bandwidths and upper frequency limits beyond 750 MHz to 1 GHz and higher. Therefore, a need exists for an improved RF choke which overcomes some of the problems and shortcomings of the prior art.